Semiconductor device including SiON gate dielectric with portions having different nitrogen concentrations

ABSTRACT

An integrated circuit (IC) includes a substrate having a top semiconductor surface including at least one MOS device including a source and a drain region spaced apart to define a channel region. A SiON gate dielectric layer that has a plurality of different N concentration portions is formed on the top semiconductor surface. A gate electrode is on the SiON layer. The plurality of different N concentration portions include (i) a bottom portion extending to the semiconductor interface having an average N concentration of &lt;2 atomic %, (ii) a bulk portion having an average N concentration &gt;10 atomic %, and (iii) a top portion on the bulk portion extending to a gate electrode interface having an average N concentration that is ≧2 atomic % less than a peak N concentration of the bulk portion.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. Nonprovisional patentapplication Ser. No. 12/710,709, filed Feb. 23, 2010, the contents ofwhich are herein incorporated by reference in its entirety.

FIELD

Disclosed embodiments relate to integrated circuits (ICs) including MOStransistors having SiON gate dielectrics.

BACKGROUND

Replacing conventional silicon oxide (e.g. SiO₂) gate dielectrics withsilicon oxynitride (e.g. SiON) layers for MOS transistors reduces gatedielectric leakage and boron (B) penetration from the gate electrodeinto the semiconductor surface which can result in Vt shifts. Aconventional method to form a SiON gate dielectric layer includesthermal oxidation of silicon to form a SiO₂ base dielectric, followed bya plasma nitridation to incorporate nitrogen (N) throughout the SiO₂dielectric, and then a thermal anneal in O₂/N₂ at around 1100° C.

This conventional method results in a relatively constant Nconcentration in the thickness direction of the SiON layer. It is knownthat as the N concentration in the bulk of the SiON layer is increased,gate leakage and boron penetration (for B doped polysilicon gates) intothe semiconductor surface decreases, and that as the N concentration atthe semiconductor interface increases, the carrier mobility at thesemiconductor surface (and thus the device transconductance (Gm)), aswell as delta threshold voltage (Vt) and negative bias temperatureinstability (NBTI) all degrade. Therefore, due to the relativelyconstant N concentration in the thickness direction of the SiON layerprovided by the conventional method, the amount of N that can beincorporated into the bulk of the SiON layer is limited by the degree ofcarrier mobility degradation due to N at the semiconductor interfacethat can be tolerated in the IC design. This situation results in atrade-off in the N concentration in the SiON layer between the amount ofleakage reduction/B blocking and the carrier mobility (and thus Gm). Asa result of this tradeoff, the N concentration selected is generally nomore than about 10-15 atomic %.

SUMMARY

This Summary is provided to comply with 37 C.F.R. §1.73, presenting asummary of the disclosed embodiments to briefly indicate the nature andsubstance of this Disclosure. It is submitted with the understandingthat it will not be used to interpret or limit the scope or meaning ofthe claims.

Disclosed embodiments provide SiON gate dielectrics that decouple the Nconcentration away from the semiconductor interface (e.g. in the bulk ofthe SiON layer) of the SiON dielectric from the N concentration at theSiON semiconductor interface. This decoupling allows the average Nconcentration at the semiconductor interface to be significantly lower(e.g. at least a factor of 2, such as a factor of >5) as compared to theaverage N concentration away from semiconductor interface tosimultaneously achieve low leakage, high B blocking, and high carriermobility at the semiconductor surface. Disclosed embodiments thusprovide SiON gate dielectrics that eliminate the conventional trade-offin the N concentration between the amount of leakage reduction/Bblocking and the carrier mobility.

In one embodiment, the SiON dielectric layer includes three different Nconcentration portions. A top portion provides a top interface with thegate electrode (e.g. polysilicon) that has a moderate average Nconcentration, a bulk portion that has the highest peak N concentration,and a bottom portion that provides a semiconductor interface with thetop semiconductor surface that has the lowest average N concentration.

The Inventors have discovered that the moderate average N concentrationat the top portion of the SiON dielectric layer can reduce Bdeactivation in the case the gate electrode comprises polysilicon tominimize poly depletion. The high N concentration including a peak Nconcentration in the bulk portion enables leakage reduction and areduction in B penetration into the semiconductor surface. The bottomportion has the lowest N concentration to maximize the carrier mobilityin the channel region, and to reduce the Vt shift and NBTI degradation.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross sectional view of a portion of an integrated circuit(IC) including a substrate having a top semiconductor surface includingat least one MOS device that includes a SiON gate dielectric layerhaving a plurality of different N concentration portions, according to adisclosed embodiment.

FIG. 2 is a graphical representation of a N concentration profile for anexemplary SiON gate dielectric layer based on High Resolution RutherfordBackscatter (HR-RBS) data obtained having a plurality of different Nconcentration portions, according to a disclosed embodiment.

FIG. 3 is a flow chart of an exemplary method for forming an ICcomprising at least one MOS device comprising a SiON gate dielectriclayer having a plurality of different N concentration portions on asubstrate wafer having a top semiconductor surface, according to adisclosed embodiment.

FIG. 4 a plot of gate leakage density (Jg (A/cm²)) versus inversionelectrical thickness (Tox, inv (Angstroms)) for a SiON gate dielectriclayer formed using a 2-step post nitridation anneal as compared to aSiON gate dielectric layer formed using a conventional single step N₂/O₂PNA, according to a disclosed embodiment.

DETAILED DESCRIPTION

Disclosed embodiments are described with reference to the attachedfigures, wherein like reference numerals are used throughout the figuresto designate similar or equivalent elements. The figures are not drawnto scale and they are provided merely to illustrate the disclosedembodiments. Several aspects are described below with reference toexample applications for illustration. It should be understood thatnumerous specific details, relationships, and methods are set forth toprovide a full understanding of the disclosed embodiments. One havingordinary skill in the relevant art, however, will readily recognize thatthe disclosed embodiments can be practiced without one or more of thespecific details or with other methods. In other instances, well-knownstructures or operations are not shown in detail to avoid obscuring thedisclosed embodiments. The disclosed embodiments are not limited by theillustrated ordering of acts or events, as some acts may occur indifferent orders and/or concurrently with other acts or events.Furthermore, not all illustrated acts or events are required toimplement a methodology in accordance with disclosed embodiments.

FIG. 1 is a cross sectional view of a portion of an integrated circuit(IC) 100 including a substrate 105 having a top semiconductor surface107 including at least one MOS device 110 that includes a SiON gatedielectric layer 120 having a plurality of different N concentrationportions, according to a disclosed embodiment. Top semiconductorinterface 108 shown in FIG. 1 represents the interface betweendielectric layer 120 and the top semiconductor surface 107. Substrate105 may include any one of a bulk silicon substrate, SiGe substrate,strain silicon substrate, an SOI substrate, or other single crystalsubstrate.

The MOS device is shown in a highly simplified form and as shownincludes a source 111 and a drain region 112 spaced apart to define achannel region 113. Features such as LDDs, spacers and additionalimplants (e.g. halo) are not shown to avoid obscuring inventive details.The SiON layer 120 has a plurality of different N concentration portionsand is formed on the top semiconductor surface 107. A gate electrode 130is on the SiON layer 120. In one embodiment, the gate electrode 130comprises polysilicon, such as P+ (e.g. B) doped for PMOS devices and N+doped for NMOS devices. In other embodiments, the gate electrode 130comprises a metal gate, such as when a replacement metal gate process isemployed, so that the gate electrode 130 can comprise metal gates suchas W/TiN, Mo, Ta, TaN, TiN or TaSi_(x)N_(y).

The plurality of different N concentration portions include (i) a bottomportion 121 extending to the semiconductor interface at the topsemiconductor surface 107 having an average N concentration of <2 atomic%, (ii) a bulk portion 122 having an average N concentration >10 atomic%, and (iii) a top portion on the bulk portion extending to a gateelectrode interface 126 having an average N concentration that is ≧2atomic % less than the peak N concentration of the bulk portion 122.

Although not shown in FIG. 1, IC 100 generally includes other activecircuitry, comprising circuit elements that generally include othertransistors including bipolar transistors, diodes, capacitors, andresistors, as well as signal lines and other conductors thatinterconnect these various circuit elements.

FIG. 2 is a graphical representation of a N concentration profile fromRBS data obtained for an exemplary SiON gate dielectric layer 120actually fabricated having a plurality of different N concentrationportions, such as SiON layer 120 shown in FIG. 1 comprising portions121, 122, 123, according to a disclosed embodiment. Each monolayer ofSiON is generally about 2.7 Angstroms thick. As used herein, eachmonolayer of SiON corresponds to between 2.5 to 3 Angstroms in SiONthickness. As shown, the bottom portion 121 has a thickness between 1and 2 monolayers (corresponding to 2.5 to 6 Angstroms thick), the bulkportion 122 has a thickness of 2 to 6 monolayers (corresponding to 5 to18 Angstroms thick), and the top portion 123 has a thickness of 1 to 2monolayers (corresponding to 2.5 to 6 Angstroms thick).

FIG. 2 is a graphical representation of a N concentration profile for anexemplary SiON gate dielectric layer based on HR-RBS data obtainedhaving a plurality of different N concentration portions, according to adisclosed embodiment. FIG. 2 evidences a N concentration of portion 121at the semiconductor interface 108 of <2 atomic %, with a Nconcentration level below the detection limit as shown. The peak Nconcentration is in the bulk portion 122 is seen to be at least 15atomic %, with about 24 atomic % shown. The top portion 123 has anaverage N concentration that can be seen to be ≧2 atomic % less than thepeak N concentration of the bulk portion 122. The (mathematical) averageN concentration throughout the SiON layer 120 is seen to be at least 8atomic %. In other embodiments, the average N concentration throughoutthe SiON layer 120 is ≧10 atomic %.

FIG. 3 is a flow chart of an exemplary method 300 for forming an ICincluding at least one MOS device comprising a SiON comprising gatedielectric having a plurality of different N concentration portions on asubstrate wafer having a top semiconductor surface, according to adisclosed embodiment. Step 301 comprises forming a 1.0-3.0 nm thicksilicon oxide layer by oxidizing the top semiconductor surface. In oneembodiment the thickness of the silicon oxide layer is 1.0 to 2.0 nm.The embodiment described relative to FIG. 2 used a nearly 2 nm siliconoxide layer. The silicon oxide layer can be grown by oxidation inambients such as O₂, O₂+H₂, N₂O, N₂O+H₂, H₂O, in a temperature range of800-1100° C., at a pressure of 0.001-100 Torr, for a time of 1-60 s.Reduced pressure (i.e. sub-atmospheric pressure) oxidation reduces theoxidation rate to limit the thickness of the silicon oxide layer.

Step 302 comprises nitriding the silicon oxide layer. The nitridationperformed in the nitriding step 302 is used to set the basic shape ofthe nitrogen concentration profile such that the N concentration in thebulk portion 122 of the SiON layer 120 is sufficiently high to provideleakage current (Jg) reduction and B blocking (for B doped silicongates), while being shallow enough to prevent nitridation of the topsemiconductor interface 108 and resulting mobility/delta Vt degradation.The nitridation can be performed in an N₂ plasma, N₂+He plasma, N₂+noble gas plasma or an NH₃-containing plasma. In another embodiment, thenitridation is performed in NH₃ or NH₃ containing ambient at atemperature of 500-1000° C., at a pressure of 0.001-100 Torr, for a timeof 1-60 s.

Step 303 comprises, after the nitriding, a multi-step post nitridationannealing (PNA) comprising a first anneal 303(a) in an inert ambient.The first anneal 303(a) is an inert anneal that can remove weakly bondedN and stabilize the remaining N in the SiON layer 120 such that it isless likely to diffuse out from the SiON layer during subsequent thermalprocessing. In one embodiment the first anneal is performed in a N₂ or anoble gas ambient at a temperature from 500-1100° C., at a pressure of0.001-760 Torr, for a time of 0.1-60 s.

The first anneal 303(a) is followed by a second anneal 303(b) in anoxidizing ambient. The second (oxidizing) anneal 303(b) can healremaining defects in the SiON layer without subjecting the dielectric atthe semiconductor interface 108 to additional oxidation in order toprevent degradation of the electrical thickness of the SiON layer. Thesecond anneal 303(b) is an oxidizing anneal that can also reduce the Nconcentration at the top portion of the SiON layer which the Inventorsherein have discovered can be helpful for polysilicon gates in order toreduce the N-content in order to avoid an increase in polysilicondepletion that can result from BN formation. In one embodiment thesecond anneal is performed in an oxygen containing gas including pure O₂gas at 500-1100° C., at a pressure of 0.001-100 Torr, for a time of0.1-120 s. In another embodiment, the first anneal and the second annealof the PNA are performed in a common chamber (e.g. single wafer rapidthermal annealer (RTA)).

Step 304 comprises depositing a gate electrode on the SiON layer. In oneembodiment, the gate electrode can comprise polysilicon. Polysilicon canbe deposited using a silicon comprising gas, such as SiH₄, Si₂H₆, Si₂Cl₆or SiH₂Cl₂, etc., at a temperature of 500-800° C., at a pressure of1-100 Torr, for a time from 10-300 s. In other embodiments, the gateelectrode can comprise a metal gate, such as W/TiN, Mo, Ta, TaN, TiN orTaSi_(x)N_(y) in the case of a replacement gate process.

In one embodiment, reduced pressure is maintained between step 302 andstep 304 at a level below 300 Torr, such as a level below 100 Torr. TheInventors have found that performing such steps in reduced pressureprevents uncontrolled N-loss due to reaction of the SiON layer withambient oxygen and also prevents the SiON layer from being contaminatedfrom adventitious carbon both of which can degrade device performancefor MOS devices in terms of electrical thickness and Vt control.Maintaining reduced pressure allows the SiON layer to be essentiallycarbon free (e.g. <0.2 atomic % C). For example, on one embodiment,forming the silicon oxide layer (step 301) takes place in a firstchamber, the substrate is transferred under reduced pressure to a secondchamber, wherein the nitriding (step 302) takes place in the secondchamber. The substrate is transferred under reduced pressure to a thirdchamber, wherein the first anneal 303(a) takes place in a third chamber.The substrate is transferred under reduced pressure to a fourth chamber,wherein the second anneal 303(b) takes place in the fourth chamber. Thesubstrate is transferred at step 304 under reduced pressure to a fifthchamber, wherein the gate electrode is deposited in the fifth chamber.

Step 305 comprises forming a source and a drain region spaced apart fromone another on opposing sides of the gate electrode to define a channelregion positioned under the gate electrode. Conventional processing canbe used for step 305, and subsequent steps to complete fabrication ofthe IC.

Embodiments of the invention can be integrated into a variety of processflows to form a variety of devices and related products. Thesemiconductor substrates may include various elements therein and/orlayers thereon. These can include barrier layers, other dielectriclayers, device structures, active elements and passive elementsincluding source regions, drain regions, bit lines, bases, emitters,collectors, conductive lines, conductive vias, etc. Moreover,embodiments of the invention can be used in a variety of processesincluding bipolar, CMOS, BiCMOS and MEMS.

EXAMPLES

Disclosed embodiments are further illustrated by the following specificExamples, which should not be construed as limiting the scope or contentof embodiments disclosed herein in any way.

FIG. 4 a plot of gate leakage density (Jg (A/cm²)) versus inversionelectrical thickness (Tox, inv (A)) for a SiON gate dielectric layerformed using a 2-step PNA compared to a SiON gate dielectric layerformed using a conventional single step N₂/O₂ PNA, according to adisclosed embodiment. It can be seen that the 2-Step PNA describedherein (inert anneal followed by an oxidizing anneal) results insuperior Jg/Tox,inv and Ion/Ioff performance as compared to the singlestep N₂/O₂ PNA (shown as “Prior Art”).

While various embodiments of the invention have been described above, itshould be understood that they have been presented by way of exampleonly, and not limitation. Numerous changes to the disclosed embodimentscan be made in accordance with the disclosure herein without departingfrom the spirit or scope of the disclosed embodiments. Thus, the breadthand scope of embodiments of the invention should not be limited by anyof the above explicitly described embodiments. Rather, the scope of theinvention should be defined in accordance with the following claims andtheir equivalents.

Although the embodiments of invention have been illustrated anddescribed with respect to one or more implementations, equivalentalterations and modifications will occur to others skilled in the artupon the reading and understanding of this specification and the annexeddrawings. In addition, while a particular feature may have beendisclosed with respect to only one of several implementations, suchfeature may be combined with one or more other features of the otherimplementations as may be desired and advantageous for any given orparticular application.

The terminology used herein is for the purpose of describing particularembodiments only and is not intended to be limiting to embodiments ofthe invention. As used herein, the singular forms “a,” “an,” and “the”are intended to include the plural forms as well, unless the contextclearly indicates otherwise. Furthermore, to the extent that the terms“including,” “includes,” “having,” “has,” “with,” or variants thereofare used in either the detailed description and/or the claims, suchterms are intended to be inclusive in a manner similar to the term“comprising.”

Unless otherwise defined, all terms (including technical and scientificterms) used herein have the same meaning as commonly understood by oneof ordinary skill in the art to which embodiments of the inventionbelongs. It will be further understood that terms, such as those definedin commonly used dictionaries, should be interpreted as having a meaningthat is consistent with their meaning in the context of the relevant artand will not be interpreted in an idealized or overly formal senseunless expressly so defined herein. The Abstract of this Disclosure isprovided to comply with 37 C.F.R. §1.72(b), requiring an abstract thatwill allow the reader to quickly ascertain the nature of the technicaldisclosure. It is submitted with the understanding that it will not beused to interpret or limit the scope or meaning of the following claims.

We claim:
 1. An integrated circuit (IC), comprising: a substrate havinga top semiconductor surface including at least one MOS device, said MOSdevice comprising: a source and a drain region spaced apart to define achannel region; a SiON gate dielectric layer having a plurality ofdifferent N concentration portions formed on said top semiconductorsurface including over said channel region; a gate electrode on saidSiON layer including over said channel region, wherein said plurality ofdifferent N concentration portions include (i) a bottom portionextending to a semiconductor interface with said top semiconductorsurface having an average N concentration of <2 atomic %, (ii) a bulkportion on said bottom portion having an average N concentration >10atomic %, and (iii) a top portion on said bulk portion extending to agate electrode interface with said gate electrode having an average Nconcentration that is ≧2 atomic % less than a peak N concentration ofsaid bulk portion.
 2. The IC of claim 1, wherein said bottom portion is1-2 monolayer thick, said bulk portion is 2-6 monolayer thick and saidpeak N concentration of said bulk portion is 20-40 atomic %, and whereinand said top portion is 1-2 monolayers thick and has an average Nconcentration of 4 to 18 atomic %.
 3. The IC of claim 1, wherein saidgate electrode comprises polysilicon.
 4. The IC of claim 1, wherein saidgate electrode comprises W/TiN, Mo, Ta, TaN, TiN or TaSi_(x)N_(y). 5.The IC of claim 1, wherein an average N concentration in said SiON layeris at least 8 atomic %, and a peak N concentration in said SiON layer isat least 15 atomic %.
 6. An integrated circuit (IC) including,comprising: a substrate having a top semiconductor surface including atleast one MOS device, said MOS device comprising: a source and a drainregion spaced apart to define a channel region; a SiON gate dielectriclayer formed on said top semiconductor surface including over saidchannel region, said SiON layer extending to a semiconductor interfacewith said top semiconductor surface; a gate electrode on said SiON layerincluding over said channel region, wherein said SiON layer extends to agate electrode interface with said gate electrode; wherein a Nconcentration at said semiconductor interface is <2 atomic %, an averageN concentration throughout said SiON layer is at least 8 atomic %, and apeak N concentration in said SiON layer is at least 15 atomic %.
 7. TheIC of claim 6, wherein said gate electrode comprises polysilicon.
 8. TheIC of claim 6, wherein said SiON layer comprise a plurality of differentN concentration portions including (i) a bottom portion extending tosaid semiconductor interface with said top semiconductor surface havingan average N concentration of <2 atomic %, (ii) a bulk portion on saidbottom portion having an average N concentration >10 atomic %, and (iii)a top portion on said bulk portion extending to said gate electrodeinterface having an average N concentration that is ≧2 atomic % lessthan a peak N concentration of said bulk portion.
 9. The IC of claim 6,wherein said bottom portion is 1-2 monolayer thick, said bulk portion is2-6 monolayer thick and wherein said peak N concentration of said bulkportion is 20-40 atomic %, and wherein said top portion is 1-2monolayers thick and has an average N concentration of 4 to 18 atomic %.